Temperature sensors that measure the temperature in the exhaust pipe of an engine may use a pair of thermocouple wires. Such a temperature sensor includes an exposed type temperature sensor, a grounded type temperature sensor and a non-grounded type temperature sensor. In the exposed type temperature sensor, a fluid (a fluid to be measured) whose temperature is measured is in a direct contact with a temperature measuring junction (a hot junction) formed by joining the tip ends of the pair of thermocouple wires to each other. In the grounded type temperature sensor, a temperature measuring junction is grounded to a sheath. In the non-grounded type temperature sensor, a temperature measuring junction is insulated from a sheath.
The exposed type temperature sensor is advantageous in that the responsiveness of measurement is good, but is disadvantageous in that the temperature measuring junction is easily damaged. The grounded type temperature sensor is advantageous in that the responsiveness of measurement is good, but is disadvantageous in that the sensor is prone to be affected by noise. The non-grounded type temperature sensor is advantageous in that the temperature measuring junction can be protected from damage and the sensor is not prone to be affected by noise, but is disadvantageous in that the responsiveness of measurement is degraded.
For example, Patent Literature 1 discloses a highly responsive thermocouple having a sheath, a pair of wires inserted into the sheath, and a domed protective cap attached to a tip-end portion of the sheath to cover the hot junction of the pair of wires. The domed protective cap has an intake hole formed on the top, and has an exhaust hole formed near the mounting portion of the cap. The domed protective cap prevents a gas current to be measured from directly colliding against the hot junction to thereby protect the hot junction from damage, and maintains high responsiveness of measurement.